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In thermodynamics, motive
power is an agency, as water or steam, used to impart motion. Generally, motive power is
defined as a natural agent, as water, steam, wind, electricity, etc., used
to impart motion to machinery; a motor; a mover. The term may also define
something, as a locomotive or a motor, which provides motive
power to a system. In current use, motive power may be
thought of as a synonym for either "work", i.e. force times distance, or "power", an
effect producing motion, depending on the context of the
discussion.

Contents

History

In 1679 physicist Denis Papin conceived the idea of using
steam to power a piston and cylinder engine, by watching a
steam release valve of a bone-digester rhythmically move up and
down. In 1698, based on Papin’s designs, mechanical designer Thomas Savery built
the first engine. The first scientific treatise on the energetics
of engines was the 1824 paper: Reflections on the Motive Power
of Fire written by French physicist Sadi Carnot.

As an example, the Newcomen engine of 1711 was able
to replace a team of 500 horses that had “powered” a wheel to pump
water out of out a mine, i.e. to “move” buckets of water vertically
out of a mine. Hence, we have the precursory model to the term
motive power. Based on this model, in 1832, Carnot defined
work as “weight lifted through a height”, being the very same
definition used to this day.

1824
definition

Carnot states, in the footnotes to his famous 1824 publication,
“We use here the expression motive power to express the
useful effect that a motor is capable of producing. This effect can
always be likened to the elevation of a weight to a certain height.
It has, as we know, as a measure, the product of the weight
multiplied by the height to which it is raised.”

In this manner, Carnot is actually referring to "motive power"
in the same manner we currently define "work". If we were
to include a unit of time in Carnot's definition, we would then
have the modern-day definition for power:

Thus Carnot's definition of motive power is not
consistent with the modern physics definition of "power", nor the
modern usage of the term.

1834
definition

In 1834, the French mining engineer Emile Clapeyron
refers to Carnot’s motive power as “mechanical action”. As an
example, during the expansion stroke of a piston engine he states
that: “the gas will have developed a quantity of mechanical action
during its expansion given by the integral of the product of the
pressure times the differential of the volume.” Clapeyron then goes
on to use graphical methods to show how this "mechanical action",
i.e. work in modern terms, could be calculated.